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SubjectRe: [PATCH 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack
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> On Jul 13, 2018, at 10:21 PM, Joerg Roedel <jroedel@suse.de> wrote:
>
>> On Fri, Jul 13, 2018 at 04:31:02PM -0700, Andy Lutomirski wrote:
>> What you're really doing is keeping it available for an extra flag.
>> Please update the comment as such. But see below.
>
> Thanks, will do.
>
>>> +.macro PARANOID_EXIT_TO_KERNEL_MODE
>>> +
>>> + /*
>>> + * Test if we entered the kernel with the entry-stack. Most
>>> + * likely we did not, because this code only runs on the
>>> + * return-to-kernel path.
>>> + */
>>> + testl $CS_FROM_ENTRY_STACK, PT_CS(%esp)
>>> + jz .Lend_\@
>>> +
>>> + /* Unlikely slow-path */
>>> +
>>> + /* Clear marker from stack-frame */
>>> + andl $(~CS_FROM_ENTRY_STACK), PT_CS(%esp)
>>> +
>>> + /* Copy the remaining task-stack contents to entry-stack */
>>> + movl %esp, %esi
>>> + movl PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %edi
>>
>> I'm confused. Why do we need any special handling here at all? How
>> could we end up with the contents of the stack frame we interrupted in
>> a corrupt state?
>>
>> I guess I don't understand why this patch is needed.
>
> The patch is needed because we can get exceptions in kernel-mode while
> we are already on user-cr3 and entry-stack. In this case we need to
> return with user-cr3 and entry-stack to the kernel too, otherwise we
> would go to user-space with kernel-cr3.
>
> So based on that, I did the above because the entry-stack is a per-cpu
> data structure and I am not sure that we always return from the exception
> on the same CPU where we got it. Therefore the path is called
> PARANOID_... :)

But we should just be able to IRET and end up right back on the entry stack where we were when we got interrupted.

On x86_64, we *definitely* can’t schedule in NMI, MCE, or #DB because we’re on a percpu stack. Are you *sure* we need this patch?

>
>
> Regards,
>
> Joerg
>
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